The present invention relates to an amorphous silicon semiconductor and a process for preparing the same.
With respect to an amorphous silicon prepared from silane (SiH.sub.4) by plasma decomposition, since D. E. Carlson et al prepared a solar cell using an amorphous silicon semiconductor in 1976 on the basis of the discovery by W. E. Spear et al in 1976 that a conductivity of amorphous silicon is greatly varied by doping with PH.sub.3 or B.sub.2 H.sub.6, a solar cell using a thin film amorphous silicon has been given attention, and various studies for improving a conversion efficiency of the solar cell have been carried out.
Hitherto there have been proposed several types of photovoltaic elements using thin films of the amorphous silicon semiconductor such as Shottky barrier type, p-i-n junction type, MIS type and hereto junction type. Among them, the Schottky barrier type, p-i-n junction type and MIS type photovoltaic elements are useful as a solar cell having a high conversion efficiency. For instance, the Schottky barrier type solar cell has a conversion efficiency of 5.5% (D. E. Carlson et al "IEEE Trans. Electron Devices" EP-24, 449 (1977)), the MIS type solar cell has a conversion efficiency of 4.8% (J. I. B. Wilson et al "Nature" 272, 152(1978)) and the p-i-n junction type solar cell has a conversion efficiency of 4.5% (Yoshihiro Hamakawa et al "Surface Sci." 86, 486(1979)).
In a process for preparing the photovoltaic element, however, a growth rate of an i-layer is low, i.e. 1 to 2 A/sec, which prevents production of a low-cost element. For resolving the defect, various attempts have been made by changing reaction conditions. For instance, there has been proposed an attempt for increasing growth rate by increasing power in glow discharge. When the power is increased, however, properties of the prepared thin film amorphous silicon become bad because the influence of bombardment of the generated plasma increases (J. C. Knights, "Appl. Phys. Lett." 35(3), 244(1979)). There has also been proposed an attempt in which pressure is adjusted and an attempt in which the temperature of the substrate is varied. However, both attempts have defects that properties, particularly electrical properties, of the prepared amorphous silicon semiconductor become low level.
An object of the present invention is to provide a process for preparing an amorphous silicon semiconductor at an increased growth rate without impairing the properties of the semiconductor.
Another object of the present invention is to provide an amorphous silicon semiconductor having good electrical and optical properties.